1. Field of the Invention
The present invention relates to a power output apparatus and a method of controlling the same. More specifically the present invention pertains to a power output apparatus for outputting power to a drive shaft and a method of controlling such a power output apparatus.
2. Description of the Related Art
Known power output apparatuses for carrying out torque conversion of power output from an engine and outputting the converted power to a drive shaft include a combination of a fluid-based torque converter with a transmission. In such a power output apparatus, the torque converter is disposed between an output shaft of the engine and a rotating shaft linked with the transmission, and transmits the power between the rotating shaft and the output shaft through a flow of the sealed fluid. Since the torque converter transmits the power through a flow of the fluid, there is a slip between the output shaft and the rotating shaft, which leads to an energy loss corresponding to the slip. The energy loss is expressed as the product of the revolving speed difference between the rotating shaft and the output shaft and the torque transmitted to the output shaft, and is consumed as heat.
In a vehicle with such a power output apparatus mounted thereon as its power source, at the time when there is a large slip between the rotating shaft and the output shaft, that is, when a significantly large power is required, for example, at the time of starting the vehicle or running the vehicle on an upward slope at a low speed, a large energy loss in the torque converter undesirably lowers the energy efficiency. Even in a stationary driving state, the efficiency of power transmission by the torque converter is not 100%, and the fuel consumption rate in the conventional power output apparatus is thereby lower than that in a manual transmission.
In order to solve such problems, the applicants of the present invention have proposed a system that does not include the fluid-based torque converter but is provided with an engine having an output shaft, a generator having a rotating shaft, a planetary gear including a carrier, a sun gear, and a ring gear respectively linked with the output shaft of the engine, the rotating shaft of the generator, and a drive shaft, and a battery connected to the generator and a motor, and outputs the power from the engine or the electric power stored in the battery to the drive shaft as a desired power (JAPANESE PATENT LAYING-OPEN GAZETTE No. 50-30223).
In the proposed power output apparatus, the power output from the engine that is driven at an arbitrary driving point is divided by means of the planetary gear into power transmitted to the generator and power transmitted to the drive shaft. In case that the power output from the engine is output to the drive shaft as the decreased revolving speed and the increased torque, the power transmitted to the generator is regenerated as electric power, which is used to drive the motor linked with the drive shaft and apply the torque to the drive shaft. In case that the power output from the engine is output to the drive shaft as the increased revolving speed and the decreased torque (that is, in the over-driver mode), on the other hand, the power transmitted to the motor is regenerated as electric power, which is used to drive the generator as a motor. Since the motor is linked with the drive shaft, in the over-drive mode, while the electric power regenerated by the motor is supplied to the generator, part of the power obtained by driving the generator is again regenerated as electric power by the motor before being output to the drive shaft by means of the planetary gear. This causes a circulation path, where part of energy circulates in the sequence of the motor, the generator, the planetary gear, and the motor. An increase in amount of circulated energy decreases the ratio of the energy of smaller loss, which is directly output from the engine to the drive shaft via the planetary gear, and thereby lowers the energy efficiency of the whole power output apparatus.